[Plumbing] Is it normal for a copper sink to shine and tarnish in places?

When I first got my copper sink, it was somewhat shiny, after a month or two, it was installed and had turned a more tarnished copper / brown. Similar to an older penny.

Some areas of the sink become less tarnished and others are still tarnished, based more or less on use. Sort of blotchy, shiny to tarnished. This seems to be related to what was recently sitting in the sink, and for how long. We wash most everything every night, but the color changes a bit.

The sink looks very nice, but mostly the base is a bit blotchy from shiny to tarnished.

Sounds normal to me. Copper is very reactive when exposed to other chemicals, or even air. Acids in some foods, combined with a bit of water, will form an electrolytic battery when trapped between the copper and a dissimilar metal.

As the others have said this is of course a naturally occurring process that copper is prone to.

There are commercially available sealants that can be used to treat the surface if you want to keep that "like new" look. But that seems counterintuitive to me -- why spend extra on a copper basin if you're not going to let it do it's thing?

I have a copper insert that I use for soap/shampoo in a tiled shower. When new it was just too shiny. So I waited for it to "age" to the point that I liked. Cleaned/dryed it up real good and used a sealer that they use for cemetery markers and used the whole can(about 6 coats). Hasn't changed color for over a year.